The present invention relates to a reactive power processing system for a current source GTO (Gate Turn Off Thyristor) invertor. In a current source GTO (Gate Turn Off Thyristor) invertor in which a plurality of GTO thyristors are used for switching elements of an inverter main circuit, no commutating circuit to forcefully turn off each GTO thyristor externally is required since the GTO thyristors have self detriggering capabilities, i.e., the GTO thyristor can be turned on and off depending on a polarity of a gate trigger signal applied to the gate terminal thereof. However, when the current source GTO inverter drives such a load as an induction motor having a reactance, a commutation surge voltage generated due to the reactance of the load may cause damages of some GTO thyristors. The commutation surge voltage is generated whenever each of the GTO thyristors is sequentially turned off.
A U.S. Pat. No. 4,580,205 issued on Apr. 1, 1986 (which corresponds to a Japanese Patent Application Unexamined Open No. Sho. 59-165970 and corresponds to an European Patent Application file No. 84 102 580.2) discloses a surge voltage clamping circuit for clamping the commutation surge voltage generated when each GTO incorporated in the current-type GTO bridge-connected invertor is turned off.
FIG. 1 shows the surge voltage clamping circuit disclosed in the above-identified Patent document.
In FIG. 1, a direct-current (DC) output derived from a power rectifier 1 including a plurality of bridge-connected thyristors (reverse blocking triode thyristors) whose gates are phase controlled is smoothed by means of a DC reactor having two inductive reactances 2A, 2B magnetically coupled to each other. The smoothed DC current id is inverted into an alternating current (AC) by means of a GTO bridge-connected invertor 3 and sent to an induction motor 4 which is a load of the invertor 3.
The invertor 3 includes six bridge-connected GTO thyristors G.sub.1 through G.sub.6 as main circuit switching elements. Each GTO thyristor G.sub.1 through G.sub.6 has a conduction interval of an electrical angle width of 120 degrees and is triggered for each pulse angle width of 60 degrees in such a conduction order as G.sub.1, G.sub.6, G.sub.3, G.sub.2, G.sub.5, and G.sub.4. Consequently, a rectangular wave alternating current having the electrical angle width of 120 degrees is outputted from the invertor 3.
In the invertor shown in FIG. 1, six times of commutations are carried out for one period (360.degree. ). The commutation surge voltage appearing on each arm of the GTO thyristors in the bridge configuration must be clamped. In addition, a reactive power in the induction motor 4 must be regenerated to the input power supply.
Therefore, a reactive power processing circuit (surge voltage clamping circuit) 6 and a fly wheel circuit 5 are disposed between the power rectifier 1 and invertor 3. The fly wheel circuit 5 includes six diodes D.sub.1 through D.sub.6 in a bridge configuration, each phase U, V, and W being connected to an intermediate alternating current input side of a pair of diodes D.sub.1 and D.sub.2, D.sub.3 and D.sub.4, and D.sub.5 and D.sub.6.
A commutation surge energy derived from the induction motor 4 is rectified by means of the fly wheel circuit 5 and is stored across a capacitor C.sub.1 of the reactive power processing circuit 6 as a charge current. When an excessive charge in the capacitor C.sub.1 occurs, two GTO thyristors G.sub.7, G.sub.8 in the processing circuit 6 are triggered to turn on so that the excessive charge current is sent to the DC side of the power rectifier 1.
The two GTO thyristors G.sub.7, G.sub.8 are simultaneously triggered to turn on when the voltage e.sub.c1 across the capacitor C.sub.1 is higher than the DC output voltage e.sub.d of the power rectifier 1 by a constant value. At this time, the power is regenerated to the DC current side of the power rectifier 1 along such a route as a reactor L.sub.r2 of cumulative windings.fwdarw.GTO thyristor G.sub.8 .fwdarw.capacitor C.sub.1 .fwdarw.GTO thyristor G.sub.7 .fwdarw.reactor L.sub.r1. In addition, when the voltage across the capacitor C.sub.1 become reduced and reaches below the DC current side of the power rectifier 1 due to the discharge occurring when the above-described regeneration operation, both GTO thyristors G.sub.7, G.sub.8 are simultaneously turned off. Thereafter, electromagnetic energies stored in the two reactors L.sub.r1, L.sub.r2 are charged across the capacitor C.sub.1 along such a route as reactor L.sub.r2 .fwdarw.diode D.sub.8 .fwdarw.capacitor C.sub.1 .fwdarw.diode D.sub.9 .fwdarw.reactor L.sub.r1.
Both reactors L.sub.r1`, L.sub.r2 are installed for the suppression of discharge current of the capacitor and the compensation for a difference between the output voltage of the power rectifier 1 and voltage across the capacitor C.sub.1. The diodes D.sub.8 and D.sub.9 are installed for the energy absorption stored in the reactors L.sub.r1 and L.sub.r2.
Hence, the reactive power processing circuit shown in FIG. 1 requires the cumulative winding reactors L.sub.r1, L.sub.r2, GTO thyristors G.sub.7, G.sub.8, and diodes D.sub.8, D.sub.9. Consequently, the number of circuit elements are increased and the circuit accordingly becomes complex. In addition, the manufacturing cost is increased.
FIG. 2 shows another reactive power processing circuit (commutation surge voltage clamping circuit) disclosed in a Conference Record of the 1986 IEEE Industry Applications Society Annual Meeting Part I Pages 521 through 526. The reactive power processing circuit 6A shown in FIG. 2 is proposed to drive the induction motor 4 in a quadrant operation mode. The reactive power processing circuit 6A is installed since only the regeneration of the motor 4 to the DC current side of the power rectifier cannot suppress the voltage across the capacitor C.sub.1 shown in FIG. 1. That is to say, a pair of GTO thyristors G.sub.9, G.sub.10 are disposed across the corresponding diodes D.sub.8, D.sub.9 so that the regeneration of the motor 4 to the alternating current power supply side via the power rectifier 1 can be achieved. Furthermore, a pair of diodes D.sub.10, D.sub.11 are disposed across the corresponding GTO thyristors G.sub.7, G.sub.8 because of the presence of the two reactors L.sub.r1, L.sub.r2.
Therefore, it follows that the more increases in the number of circuit elements and in the manufacturing cost.
In addition, a time constant between each of the reactors L.sub.r1, L.sub.r2 and capacitor C.sub.1 causes a frequency range to be processed to be limited and a variable speed control for a high speed induction motor becomes difficult.